GENERAL DESIGN |
The following components of a generic GPS receiver can be identified (see Figure below):
Antenna and Preamplifier Electronics: Antennas
used for GPS receivers have broadbeam characteristics, thus they do
not have to be pointed to the signal source as in the case of satellite
TV receiving dishes. The antennas are rather compact and may be tripod or
vehicle mounted. In the case of GPS surveying applications, the actual position
determined is the electronic phase centre, which then has to be correctly
related to the ground mark being coordinated.
Radio Frequency Section & Computer: The
RF section contains the signal processing electronics. There is, in
addition, an onboard microprocessor to not only carry out computations such
as extracting the ephemerides, determining the elevation/azimuth of the
satellites, etc., but also to control the tracking and measurement function
within digital tracking circuits. Different receiver types may use different
techniques to process the signal, but all civilian navigation receivers
are of the so-called "code-correlating" variety.
Control or Interface Unit: The interface unit
enables the operator to control and query the functions of the microprocessor.
Its size and type varies greatly, ranging from a handheld keyboard unit
to a series of soft keys surrounding an LCD screen fixed to the front of
the receiver "box". There are also many types of electronic interfaces
for specialised applications, including the digital map displays, computer
generated speech output, and interfaces to other instrumentation.
Recording Device: In the case of GPS surveying
receivers, the measured data must be stored on some suitable medium
for subsequent baseline computation ("post-processing").
A variety of data storage devices were utilised in the past, including cassette
and tape recorders, floppy disks and computer tapes, etc. Nowadays almost
all receivers utilise either solid state (RAM) memory or removable "memory
cards", but they can also be connected directly to a computer and the
data recorded directly onto the harddisk.
Power Supply: Transportable GPS receivers
use low voltage DC power. The trend towards more energy efficient instrumentation
is a strong one and GPS surveying receivers are flexible enough to operate
from a number of power sources, including internal NiCad or Lithium batteries,
camcorder batteries, as well as external batteries such as wet cell car
batteries, or from mains power.
Although there is a great variety of GPS hardware, one basic classification system for hardware is based on the type of observable that is tracked: (a) civilian navigation receivers using the C/A code on the L1 frequency, (b) military navigation receivers using the P (Y) code on both L-band frequencies, (c) single frequency (L1) carrier phase tracking receivers, and (d) dual-frequency carrier phase tracking receivers. The latter two are discussed further in section 4.2.3.
The antenna and RF technology components are discussed briefly below. For further details the reader is referred to WELLS et al (1987) and LANGLEY (1991a).
The generic GPS surveying receiver. (WELLS
et al, 1987)
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© Chris Rizos, SNAP-UNSW, 1999